Watermarking is a technique utilized to tag protected contents. This tag is used to detect unauthorized use or illegal copies of the protected contents. The watermarking technique consists to embed a digital mark into the content. From the viewpoint of appearance, the embedded digital mark can be either invisible or visible. From the viewpoint of the information-nature, this digital mark could be a unique value or a generic symbol depending on what the owner of the content wants to monitor. Especially in the case of the unique value, the watermarking insertion should be done in the final client device because the mark shall contain an identifier of this device.
The goal of the watermarking technique is to hide as much as possible the embedded mark/hidden information by spreading one of its representations into the content. To ensure the discretion of the mark insertion without any compromise on the quality, the insertion requires generally a complex even exhibitive computing capacity for given devices. Consequently, the complete computation is often divided into pre- and post-process. The pre-process performs most of the heavy operations which generates some cues called Watermarking Metadata (WMDs) which aid the much lighter post process to effectively insert the mark. “Where to modify”, “how to modify” is typically the information contained in the WMDs. This could be in the form of a file of pre-marking records, each containing a content index in the form of an address of the block to be marked and with at least one alternate value. At the client device, each record is processed and the alternate value is selected (or not selected) according to the bit of the identifier to be included into the content.
Therefore the secrecy of the WMDs as well as their insertion shall be ensured, especially when the post-process is performed, inside the client device, to avoid the ability of filtering, removing and/or cropping them. At the client device, existing digital watermarking processing techniques are generally controlled by application software (HOST CPU) of the device meaning that in some cases the unprotected and still unmarked content could be accessible by the software of the client device. Therefore the security of the digital watermarking processing depends on the security of the software running on the device i.e. how easy it is to modify and then to by-pass the digital watermarking processing when the device is successfully attacked or just open (no software authentication).
The document US2010/128871 describes a solution in which a secondary stream is generated comprising the data allowing to reconstruct the main stream and at the same time to marl the reconstructed stream. As a consequence, the same stream contains the scrambling data and the watermarking data. At the reception, this stream is processed as one set of data to be replaced in the modified stream.
The document EP 2 391 125 describes a solution to allow an individual marking (at the reception device) based on a stream common to all devices. The control object contains the original value, an alternate value and a location. The security unit determines a mathematical operation to be applied on the alternate value to retrieve the original value. The mathematical operation is changed according to an internal parameter of the reception device so that the mathematical operation will be unique per device, allowing to track this device if the stream of descrambling data is analyzed.